1. Field of the Invention
Embodiments of the present invention relate to a sucker rod connection, and more particularly, a sucker rod connection with improved fatigue resistance and a method for achieving the same.
2. Description of the Related Art
Sucker rods are rods, typically made of steel or other materials, used to make up the mechanical assembly between the surface and downhole components of a rod pumping system. Sucker rods are conventionally about 25 to 30 ft (about 7 to 9 m) long and have threaded pins at ends of an elongate bar to enable the downhole components to be run and retrieved easily. Sucker rods can be connected with a threaded box or other threaded connection engaging the threaded pin. Typical diameters for sucker rods are ⅝″, ¾″, ⅞″, 1″ and 1⅛″.
The problem of sucker rod failure is particularly severe in 22.22 mm (⅞″) diameter, grade D, API-11B rods. Most failures occur in the rod in the area of the last thread engaged of the pin. In the prior art, for example with a 22.22 mm (⅞″) diameter, grade D, API-11B rod, the thread of the rod is cylindrical, and the diameter of the pin thread is a little smaller than the diameter of the box thread. As shown in FIG. 1A and FIG. 3, only the load flanks of the pin and box come in contact when making up the connection. In other words, there is no diametrical interference, and therefore to prevent disengagement it is necessary to introduce in the connection high deformation between the shoulder of the box and the shoulder of the pin after they come in contact. This deformation creates high axial tension in the pin and compression in the box. This tension in the pin, because of the make up, is added to the cyclical external tension (fatigue) which appears when the rod is working on pumping oil at the well. As a result, very often the tension goes above the resistance allowed of the steel and breaks the pin body in the area of the last engaged thread.